This expression gives us the rest-density of electricity (see §8 and §4). Then

(61)

s+(ws¯)w{\displaystyle s+(w{\bar {s}})w}

represents a space-time vector of the 1st kind, which since ww¯=1{\displaystyle w{\bar {w}}=1}, is normal to w, and which I may call the rest-current. Let us now conceive of the first three component of this vector as the x-, y-, z co-ordinates of the space-vector, then the component in the direction of w{\displaystyle {\mathfrak {w}}} is